Wireless communications devices, for instance, mobile phones have become popular remarkably in recent years with their functions and services improved increasingly. Taking a mobile phone as an example, there are various systems for mobile phones, for instance, EGSM (extended global system for mobile communications) and DCS 1800 (digital cellular system 1800) systems widely used mostly in Europe, a PCS (personal communications services) system used in the U.S., and a PDC (personal digital cellular) system used in Japan. In a mobile phone used in such systems, it is necessary to prevent part of a transmitting output power from being reflected by the variation of impedance, etc. of an antenna, an amplifier from being damaged by this reflected electric power, and a signal of an adjacent channel from entering from an antenna to cause mutual modulation. It is also regulated, for instance, in PDC, etc. that a signal for controlling a transmitting output is sent from a base station to mobile phones to control the transmitting output power of each mobile phone.
Therefore, in a mobile phone having a transmitting circuit means having a structure shown in FIG. 18, a high-frequency signal from a modulation circuit (not shown) is amplified by an amplifier 1, and an output proportional to the high-frequency signal is taken out by a directional coupler 2 and supplied to an automatic gain control circuit 7 to control the output power of the amplifier 1. Also, a non-reciprocal circuit device (isolator) 3 placed on the downstream side of the directional coupler 2 prevents a reflected wave generated by the mismatching, etc. of characteristic impedance and line impedance in each part (an antenna 6, a low-pass filter 4 and a duplexer 5) from entering into the amplifier 1.
FIG. 19 is an exploded perspective view showing a conventional non-reciprocal circuit device. This non-reciprocal circuit device comprises a central conductor assembly 10, a resin case 12, dielectric bodies 50a, 50b, 50c constituting load capacitors, a permanent magnet 9, and metal cases 7, 8. The central conductor assembly 10 comprises an integral central conductor member constituted by a ground electrode formed by a thin copper plate and central conductors 14a, 14b, 14c radially extending therefrom in three directions, and a disc-shaped garnet (magnetic body) 13, the central conductor member encircling the disc-shaped garnet (magnetic body) 13, and the central conductors 14a, 14b, 14c being folded and crossing at 120° with mutual insulation at a center on the upper surface of the garnet (magnetic body) 13. The central conductor assembly 10 is placed in a recess 15 substantially at a center of the resin case 12, and the dielectric bodies 50a, 50b, 50c are placed in three rectangular recesses around the recess 15. The ground electrode of the central conductor member is soldered to a ground plate of the resin case 12, and the central conductors 14a, 14b, 14c (input/output electrodes) of the central conductor member are soldered to external electrodes of the dielectric bodies 50a, 50b, 50c on their upper surfaces. The permanent magnet 9 for applying a DC magnetic field to the central conductors 14a, 14b, 14c on the garnet 13 is placed above the central conductor assembly 10. These parts are entirely received in a pair of upper and lower metal cases 7, 8. A pair of upper and lower metal cases 7, 8 also serve as magnetic yokes to constitute a magnetic circuit, providing a non-reciprocal circuit device having an outer size of 5 mm×5 mm×1.7-2.0 mm.
However, when mobile phone having such a structure comprises a transmitting circuit means, into which the directional coupler 2, coupling capacitors, the non-reciprocal circuit device 3 and the low-pass filter 4 are incorporated as separate parts, there arise disadvantages described below.
Demand has been increasingly higher for mobile phones to make areas occupied by the directional coupler 2, the low-pass filter 4 and the amplifier 1 as small as possible for miniaturization, and to reduce the cost per function and the number of parts as much as possible for price reduction. Under such demand, though areas occupied by the directional coupler 2, the non-reciprocal circuit device 3, the low-pass filter 4 and the amplifier 1 can be reduced by miniaturizing these parts, such means has its own limits. In addition, if the non-reciprocal circuit device 3 is tried to be miniaturized simply by the miniaturization of the central conductor assembly 10 and the dielectric bodies 50a, 50b, 50c, there would arise the following problems: If the central conductor assembly 1 is miniaturized, the non-reciprocal circuit device deviates from the optimally operable size as a magnetic body. In addition, if a dielectric material having a high dielectric constant is used to miniaturize the dielectric body, loss by the dielectric material increases relatively, resulting in deterioration in electric characteristics as the non-reciprocal circuit device.
If miniaturized, the directional coupler 2 has extremely deteriorated isolation characteristics. Because of the deterioration of isolation characteristics, directivity, one of the important characteristics of the directional coupler 2, cannot sufficiently be obtained. As a result, part or all of the reflected wave in a direction opposite to the traveling direction of the transmitting signal flows into the coupling terminal P5, failing to obtain the desired degree of coupling. Further, a new matching circuit should sometimes be added to achieve impedance matching between the directional coupler and the non-reciprocal circuit device. Incidentally, the directivity is determined by the following equation:Directivity=isolation between output terminal and coupling terminal−amount of coupling,which should be at least 10 dB or more.
Further, the directional coupler 2 suffers from an insertion loss, which mainly comprises coupling loss and conductor loss, and the non-reciprocal circuit device 3 and the low-pass filter 4 have insertion loss. Accordingly, when they are used as separate parts, the loss of each part is accumulated, resulting in large loss in the overall transmitting circuit means. Loss in the transmitting circuit means leads to increase in power consumption, and this loss is not ignorable for mobile phones having limited battery capacities.
To solve such problems, Japanese Patent Laid-Open No. 9-270608 proposes that output in proportion to a high-frequency signal is taken out from a capacitor (output-detecting capacitance) branched from the input terminal of an isolator, that the output is supplied to an automatic gain control circuit to control the output power of an amplifier, and that the output-detecting capacitor is formed in an integral laminate constituted by laminating dielectric sheets together with load capacitors of the isolator.
However, when the output-detecting capacitance is used, sufficient directivity cannot be obtained due to the influence of parasitic capacitance. Therefore, unless an output-detecting capacitor designed by taking interference between electrode patterns into sufficient consideration is formed in the laminate, the desired coupling would not be obtained.
When the coupling of 20 dB is sought, the output-detecting capacitance should be as small as 0.15 pF, resulting in difficulty in control, and large variation in the coupling due to unevenness in production and parasitic capacitance. In addition, the interference between electrode patterns makes further miniaturization substantially difficult.